Current control schemes are widely used in switching mode power supplies. Switching mode power supplies with adaptive voltage position (“AVP”) or over current protection (OCP) need to accurately sense an output current. AVP is one way to meet strict transient requirements on switching mode power supplies. The basic principle of the AVP control scheme is illustrated in FIG. 1, in which the ordinate represents an output voltage VO, and the abscissa represents an output current IO. It is believed that the relationship between the output voltage VO and the output current IO is as follows:VO=VSET−R*IO  (1)where the coefficient R represents the variation slope, and both VSET and R are constant.
The AVP current loop is fast with respect to the outer voltage loop in the system so that it creates an output resistance and keeps the system stable. Prior art AVP control senses the output current by sensing the inductor direct current resistance (“DCR”) current. FIG. 2A illustrates one inductor DCR current sensing method in accordance with the prior art. As shown in FIG. 2A, the switching mode power supply senses the inductor current by a capacitor CS and a resistor RS connected to an inductor L and its DCR as shown. When the time constant of CS×RS matches the time constant of L/DCR, the voltage of capacitor is equal to the voltage across the DCR, i.e., VCS=iL×DCR. Because the DCR can be obtained when the inductor is selected, the needed inductor current is proportional to VCS/DCR.
However, the controller of such switching mode power supply needs two pins to receive the sensed current. In addition, the sensed current can be affected by the DCR at varied temperatures. FIG. 2B illustrates waveforms of the inductor DCR current IL, the sample pulses Spulse, and the sensed current IS in a switching mode power supply that uses middle current sensing. The middle current sensing technique senses the current by sampling the current in the midpoint of the low-side power switch on time and holding the sampled current until the next cycle. The current in the midpoint of the low-side power switch on time is the average inductor DCR current, or the output current. However, the sample-and-hold process generates a large delay in the feedback loop that can degrade transient performance.